Water-absorbing resin has a property of absorbing a large amount of water, and is used as a material constituting sanitary goods such as paper diapers, sanitary napkins and incontinence pads, a water-retention agent for soil, a water sealing agent, a dew condensation preventing agent, a freshness keeping agent, a solvent dehydrating agent, a water-absorbing sheet of food, etc. As above, since the water-absorbing resin is widely used in the industry, the demand thereof is increasing more and more.
Especially in the case of sanitary goods such as paper diapers, etc., in order to reduce the thickness of the goods, the amount of pulp tends to be reduced and the amount of water-absorbing resin used tends to be increased. Moreover, since diapers became widespread worldwide, the demand thereof is increasing more and more.
Conventionally, by slightly crosslinking a hydrophilic polymer (crosslinking is usually carried out during polymerization), water-absorbing resin having a water insoluble property and a water swelling property is manufactured normally in the form of powdery particles. Examples of the water-absorbing resin are (i) a crosslinked polyacrylic acid partially neutralized product, (ii) hydrolysate of starch-acrylonitrile copolymer, (iii) a neutralized product of a starch-acrylic acid graft polymer, (iv) a saponified product of vinyl acetate-acrylic ester copolymer, (v) hydrolysate of acrylonitrile copolymer or acrylamide copolymer, (vi) crosslinked acrylonitrile copolymer, (vii) crosslinked acrylamide copolymer, (viii) crosslinked carboxymethylcellulose, (ix) crosslinked copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS), (x) crosslinked polyethylene oxide, (xi) crosslinked polyallylamine, and (xii) crosslinked polyethylenimine. However, in recent years, since there is a demand for high functions of the water-absorbing resin as improvements of performances of diapers, the method for manufacturing the water-absorbing resin has been improved variously. For example, by, in addition to the internal crosslinking of the powder, surface crosslinking (secondary crosslinking) the resin subjected to polymerization, a crosslink density gradient is given to the inside and surface of the resin. This improves the water-absorbing speed, liquid permeability and absorbency against pressure of the water-absorbing resin.
Then, in the case of sanitary goods such as paper diapers, etc., in order to reduce the thickness of the goods, there is a need for water-absorbing resin having high absorbency against pressure. Generally, the smaller the amount of fine powder (having a particle diameter of 150 μm or less) contained in the water-absorbing resin is, the better. This is because the fine powder causes clogging in water-absorbing goods such as diapers, and this becomes a factor of deteriorating liquid permeability. Such fine powder easily absorbs moisture, so that it becomes a factor of clogging in a process of manufacturing diapers, and moreover, it is not preferable for users in light of safety and health.
Disclosed as a method for manufacturing water-absorbing resin, the amount of fine powder of which is small, which has high absorbency against pressure, etc. is a method for (i) adding a crosslinking agent to dried water-absorbing resin powder, and (ii) when crosslinking the vicinity of the surface of the power while crushing the mixture, using water-absorbing resin coarse particles whose weight average particle diameters are from 200 μm to 1,000 μm (Document 1). The invention of Document 1 is as follows: When the water-absorbing resin has small particle diameters, the water-absorbing resin forms fish eyes by contacting with aqueous liquid, and an absorption speed deteriorates; Therefore, the particle diameter of the water-absorbing resin power before surface crosslinking is controlled to be large (coarse particles), and further, while crushing at least part of the particles, the vicinity of the surface is crosslinked; Thus, obtained is a water-absorbing agent, the amount of fine powder of which is small, and which has high absorbency against pressure and liquid permeability under pressure.
Moreover, disclosed are (i) a method for manufacturing the water-absorbing resin which method is characterized in that when surface crosslinking the water-absorbing resin, a crosslinking reaction is completed at 90° C. to 250° C. by using a horizontal stirring drier, a rotary drier, a disc drier, a mix drier, a fluidized bed dryer, a through-flow dryer, an infrared dryer, etc. (Document 2), and (ii) a method for manufacturing the water-absorbing resin which method is characterized in that a surface crosslinking agent is added to an internal crosslinked polymer prepared by sieve classification so that its particles are from 150 μm to 850 μm, and the resulting mixture is heated by a paddle mixer so as to be surface crosslinked (Document 3).
However, there is a problem with the techniques of Documents 1 to 3 in that a large amount of coarse particles (aggregate of the water-absorbing resin) remain in the water-absorbing resin. When the coarse particles remains in the water-absorbing resin, the sense of use of the product such as diapers is not good. Therefore, it is necessary to remove those coarse particles. However, it is necessary to carry out, for example, classification to remove the coarse particles. Moreover, throwing away the classified coarse particles is economically disadvantageous. Meanwhile, crushing the coarse particles generates fine powder.
Here, disclosed is a method for heating a mixture of the water-absorbing resin and a surface crosslinking agent-containing aqueous solution by using a stirring dryer including stirring boards having scraping blades or a stirring dryer including crushing means between stirring boards, so as to complete the surface crosslinking while suppressing the generation of the coarse particles (Document 4).
Moreover, disclosed as a method for carrying out a surface crosslinking treatment of the water-absorbing resin by using a stirrer including stirring boards is a method for manufacturing the water-absorbing resin which method is characterized in that the water-absorbing resin subjected to the heat treatment is stirred and cooled down under a stream of air by using a stirring cooler including stirring boards for the purpose of sufficiently achieving the effect of improving physical properties by the surface crosslinking on an industrial scale (Document 5).
Further, Document 6 discloses a method for (i) adding aqueous liquid containing a surface crosslinking agent to water-absorbing resin powder obtained through steps such as polymerization of monomers, heating, drying, cooling, and crushing, and (ii) heating and drying the mixture, so as to surface crosslink the water-absorbing resin powder.
Moreover, Document 7 discloses a method for treating with heat more than once the water-absorbing resin subjected to the surface treatment, for the purpose of delaying free water absorption. Moreover, Document 8 discloses that an expansion strength under pressure can be improved by repeating twice a step of adding surface treatment agent-containing liquid to the water-absorbing resin and heating this mixture.
[Document 1]
Japanese Unexamined Patent Publication 11-302391 (Tokukaihei 11-302391, published on Nov. 2, 1999)
[Document 2]
Japanese Unexamined Patent Publication 4-214734 (Tokukaihei 4-214734, published on Aug. 5, 1992)
[Document 3]
Published Japanese Translation of PCT International Publication for Patent Application 2002-515079 (Tokuhyo 2002-515079, published on May 21, 2002)
[Document 4]
Japanese Unexamined Patent Publication 2004-352941 (Tokukai 2004-352941, published on Dec. 16, 2004)
[Document 5]
Japanese Unexamined Patent Publication 2004-300425 (Tokukai 2004-300425, published on Oct. 28, 2004)
[Document 6]
Japanese Unexamined Patent Publication 2002-121291 (Tokukai 2002-121291, published on Apr. 23, 2002)
[Document 7]
Published Japanese Translation of PCT International Publication for Patent Application 2003-503554 (Tokuhyo 2003-503554, published on Jan. 28, 2003)
[Document 8]
Japanese Patent No. 2847113 (registered on Nov. 6, 1998) (Kyohyo (National Publication of Translated Version) No.: Published Japanese Translation of PCT International Publication for Patent Application 9-502221 (Tokuhyohei 9-502221, published on Mar. 4, 1997))
As disclosed in Documents 4 and 5, conventionally, in the case of improving the functionality of the water-absorbing resin by the surface treatment, it is believed that it is effective to use a stirrer such as a stirring dryer or stirring cooler including stirring boards in light of improvement of miscibility. Meanwhile, in the case of using the stirrer as disclosed in the techniques of Documents 4 and 5, there is still such a problem that a large amount of fine powder is generated during the surface crosslinking treatment. If a large amount of fine powder is generated, a surface crosslinked layer tends to be destroyed. Thus, a large amount of fine power adversely affects the physical properties of the water-absorbing resin.
The above problem may be caused because the shape of the stirring board is not appropriate for stirring a hydrophilic crosslinked polymer. That is, in conventional technologies, the stirring board is used to improve the miscibility, and the shape thereof is determined to improve the miscibility. Therefore, conventionally, there is no idea of using the stirring board to suppress the generation of the fine powder. That is, the shape of the stirring board is not determined to suppress the generation of the fine powder.
For example, since the difference between the maximum thickness and minimum thickness of the stirring board used in conventional techniques is large, too much pressure is applied to the powder of the hydrophilic crosslinked polymer when the stirring board breaks into the powder, and the powder is compressed. Thus, the density of the powder is increased, and the water-absorbing resin is damaged mechanically during the surface crosslinking treatment. As a result, the amount of fine powder may increase.
Moreover, in addition to the improvement of the functions of the water-absorbing resin, there is a problem of how to effectively massproduce the water-absorbing resin without deteriorating its performances to keep up with a growing demand for the water-absorbing resin in recent years. As disclosed in Documents 7 and 8, a method for improving the functionality by the surface treatment has already been suggested. However, the deterioration of the physical properties of the water-absorbing resin due to damage, etc. to the water-absorbing resin by the heat treatment device including stirring blades has not been discussed in light of the surface treatment, although this deterioration is problematic when scaling up the manufacture of the water-absorbing resin.
Here, as a method for increasing the amount of production of the water-absorbing resin, it may be possible to use a large heat treatment device that is simply a larger version of an existing heat treatment device. However, in the large heat treatment device, the heat transfer area with respect to the effective volume is small. Therefore, in order to keep the temperature of the water-absorbing resin within a certain range and cause a desired surface crosslinking reaction, it is necessary to extend a time for the water-absorbing resin to stay in the above device. Then, if a time for stirring the water-absorbing resin by the stirring board during the surface crosslinking treatment becomes long, the water-absorbing resin is damaged mechanically. As a result, the amount of fine powder increases, and the physical properties of the water-absorbing resin deteriorate.
That is, conventionally, there is no finding regarding what stirrer can suppress the generation of the fine powder and manufacture high-performance water-absorbing resin.
In order to avoid (i) the generation of the fine powder which generation is caused due to the increase in size of a device and (ii) the deterioration of the physical properties due to the generation of the fine powder, it may be possible to simply arrange a plurality of small heat treatment devices in parallel. However, since the numbers of accessory devices and pipings provided in front of and behind each heat treatment device increase by arranging a plurality of heat treatment devices, the cost increases. Moreover, since it is necessary to control a plurality of heat treatment devices, the efficiency deteriorates.
Here, there is a demand for a method for surface crosslinking the water-absorbing resin which method can effectively massproduce the water-absorbing resin on an industrial scale and can maintain the performances of the water-absorbing resin.